Electrons may be the glue in cuprate superconductors

Conventional superconductors are enabled by vibrations of the crystal lattice …

Artist's impression of a photon reflecting off the surface of a cuprate superconductor. The different possible mechanisms for superconductivity are exhibited in the different ways the material responds to the light.

Photograph by Claudio Giannetti

Copper oxide-based superconductors were discovered in 1986. Known as cuprate or high-Tc (for "high critical temperature") superconductors, these materials have a much higher temperature for the transition to zero resistance. But they have proven challenging to explain, since they don't behave as conventional superconductors do. While cuprate superconductors seem to conduct current via paired electrons like conventional superconductors, 26 years later, we still don't know how those pairs are formed.

A new optical examination of a bismuth-based cuprate superconductor has demonstrated that electronic excitations may be the primary driver of the superconducting transition. As described by S. Dal Conte et al. in a new Science paper, the complex interactions between electrons give rise to special quasiparticles. These are states that act as a kind of "glue" between electrons, allowing them to form the pairs that carry the superconducting current. The quasiparticle excitations are sufficient to explain the relatively high temperature of transition between the insulating state and the superconducting state.

Superconductors are materials with exactly zero electrical resistance: once started, a current will be sustained indefinitely in a superconductor, with no decay. The standard theory for this feat is that it's accomplished by coupling two electrons together into a Cooper pair, which can flow freely through the material in a way individual electrons are unable to do. In conventional superconductors, Cooper pairs arise through vibrations of the underlying crystal lattice that are called phonons (this is also the mechanism that produces sound in solids).

Cuprate superconductors also appear to use Cooper pairs to make the current flow, but because the materials have a very complex electronic structure, the phonon explanation hasn't worked, at least not as the sole mechanism. Other ways of creating Cooper pairs have been proposed, including confined loops of current, fluctuations in relative spins of electrons, and strong coupling between phonons. Which explanation—or which set—is responsible hasn't been determined.

In their recent experiment, Dal Conte et al. used optical spectroscopy: shining ultra-short pulses of light with a wide range of energies onto a bismuth-based superconducting material. (The specific material is Bi2Sr2Ca0.92Y0.08Cu2O2+δ, where δ is an adjustable amount of oxygen that provides the charge carriers that make superconducting possible.) By measuring the amount of light reflected back as a function of time and determining the energy difference between the emitted and reflected light, they were able to separate the contributions from phonons and other sources.

Because phonons are lattice vibrations, they react to the incoming light more slowly than the electronic excitations do. In their spectroscopic analysis, Dal Conte et al. saw a fairly rapid response, indicating that it is the quasiparticles arising from electronic interactions, which implies current loops and/or spin fluctuations. While phonons and their interactions may still be part of the superconducting mechanism, their role appears to be comparatively minor.

The researchers were unable to determine precisely which electronic excitations are the glue binding Cooper pairs together. However, being able to state that it is electrons rather than phonons that mediate the superconducting transition, at least in the bismuth-based cuprate material they studied, is a significant find. Future experiments along these lines should further tease apart the contributions from the various possible mechanisms, as should applying the methods to iron-based superconductors. The mystery of high-temperature superconductivity hasn't been solved yet, but the clues are narrowing things down.